27.5 Fork Offset

So the new Fox 34 27.5 forks have a 44mm offset. The X-Fusion Slants reportedly have a 39mm offset. This seems like a fair difference and I wonder what kind of a difference this would make in handling, if any?

So the new Fox 34 27.5 forks have a 44mm offset. The X-Fusion Slants reportedly have a 39mm offset. This seems like a fair difference and I wonder what kind of a difference this would make in handling, if any?

Assuming all else is equal, (HTA, A2C, wheel radius, etc) you will have a greater amount of mechanical trail with the X-Fusion fork than the Fox. (less offset = more trail)

This could be a good or bad thing, but it completely depends on the class of bike and to some extent, personal preference. For a DH race bike more trail is generally a good thing; greater stability at high speed. For a XC race bike more trail will typically be viewed as a negative creating sluggish, maybe even excessively 'floppy' handling.

There are probably people who can explain this much more eloquently than I just did, but those are the basics.

Cheers,
KP

Last edited by Kirk Pacenti; 12-20-2012 at 07:28 AM.

If you like my products and services tell everyone. If you don't, tell me - kirk(at)pacenticycledesign.com

I am talking out of my azz here. I am no bike designer just some schmuck w/ experience w/ buying stuff so..
I "feel" longer rakes go well w/ longer stems as in old school mtbs. as soon as susp forks came into play top tubes grew longer and stems went shorter. I don't mind a 135mm stem on a a fat chance wicked w/a 45mm rake fork. My other bike has a marz and the longer stem coupled w/ the shorter rake gives a flip flop feeling.

So... all other things being equal... the extra offset on the Fox will allow you to have a slacker head-angle, by roughly 1 degree. The lack of offset on the first gen of 29er forks was why 29er "sucked", as the head angle need to be steepened to get the handling "sharp", which on smaller sizes mean toe-overlap. "G2" offset allowed designs to go back to "more normal" head angles, which fixed up the handling of 29ers that are designed for such forks.

So... all other things being equal... the extra offset on the Fox will allow you to have a slacker head-angle, by roughly 1 degree. The lack of offset on the first gen of 29er forks was why 29er "sucked", as the head angle need to be steepened to get the handling "sharp", which on smaller sizes mean toe-overlap. "G2" offset allowed designs to go back to "more normal" head angles, which fixed up the handling of 29ers that are designed for such forks.

That's a useful tool and helps one see how changing one value affects other values.

But from a Designer's pov, HTA is meaningless... A Designer would start with the desired mechanical trail for the type bike (XC,TR,DH,etc), work within the constraints of fork offset and wheel radius and make the HTA a resultant dimension.

I would presume that the OP will be putting one of these forks on an existing bike. In which case, I think using the actual HTA of his frame, but changing the offset value would make for a more meaningful comparison.

Cheers,
KP

Last edited by Kirk Pacenti; 12-20-2012 at 02:10 PM.

If you like my products and services tell everyone. If you don't, tell me - kirk(at)pacenticycledesign.com

Agree with Stanley's assessment mostly, but it's worth mention that G2 should be called "J2" for Jeff Jones:
Jones, and later on Trek/Fisher and the slightly more relaxed G2 version (51mm offset) found many more reasons to run a longer offset than for concerns about toe-overlap, and said benefits are summarized well here:G2 Geometry - YouTube

As well, check out JJ's old skool short stays. Genius is what genius does.

It would be great if you could plug in a know AC and HA, then enter in a new AC and offset and it will tell the difference in trail. Because the different AC will affect head angle and I dont know how to calculate that without putting the new fork on and measuring.

My specific question: does the increased offset of the 650b Fox 34 160 over the 26" 36 160 fully compensate for the increased in AC of the 650b ( and hence slacker head angle). It would also be interesting to see the how that number compares to my bike in its 26" mode (My wheels were smaller but I had my fork 10mm higher - each having opposite effect on trail)